Why the Same Degree Arrives at Two Different Times
The fence explained why a body reads a different degree in the two frames. This page measures a consequence of that, and the consequence is larger than the size of the tilt might suggest.
A body is not a fixed dot. It is travelling along the fence. So if the two frames disagree about which degree it is at, they necessarily disagree about when it reaches any given degree.
The experiment
The cleanest way to see it is to search for the crossing of a degree you choose yourself. Give a body, give a degree between 0 and 360, and ask for the moment the body crosses it.
Then change one setting: keep the same body, degree and date range, move the reference plane from Ecliptic to Equatorial, and hold everything else still.
The table shows the results, with times local to a New York location; the Sun's row is the 2025 equinox, 09:01 UT.
| Body | Degree | Ecliptic latitude | Ecliptic crossing | Equatorial crossing | Difference |
|---|---|---|---|---|---|
| Sun | 0° | −0.0002° | 2025-03-20 05:01:28 | 2025-03-20 05:01:21 | 7 seconds |
| Moon | 0° | +5.23° | 2030-01-09 14:46:38 | 2030-01-09 19:11:10 | 4h 25m |
| Mercury | 0° | −2.59° | 2026-04-14 23:21:29 | 2026-04-14 04:10:41 | 19 hours |
| Pluto | 180° | +15.65° | 1971-10-05 | 1968-10-04 | 3 years |
Those are real runs, not estimates. Pluto reaches 0° Libra in the autumn of 1971 by one measurement and the autumn of 1968 by the other.
The Sun is the control
Look at the first row before anything else.
The Sun's ecliptic latitude is two ten-thousandths of a degree — zero, to any precision that matters. And its two crossings agree to within seven seconds, which is the residue of a body that is very nearly but not exactly on the ecliptic.
That row is what makes the rest provable rather than merely asserted. If the difference were an artefact — a rounding error, a bug, a units mix-up — it would show up for the Sun as well, and it does not. The effect appears exactly when the body has latitude, and in proportion to it.
This is what the fence predicted. The Sun lies on the fence, and at this degree it lies on the hinge itself; tilting a fence about its hinge does not move something that lies on the hinge.
Why Mercury's difference is larger than the Moon's
The third row looks wrong at first. Mercury has half the Moon's latitude and four times the time difference.
The reason is that a time difference is an angular offset divided by a speed. Both bodies are displaced along the fence by roughly four-tenths of their latitude — about 2.1° for the Moon and 1.0° for Mercury — but the Moon covers its 2.1° in four hours, while Mercury needs the better part of a day to cover 1.0°.
Angular offset comes from latitude. Time difference comes from offset divided by speed.
That is why the slow outer bodies show the largest differences. Pluto's 15.65° of latitude displaces it by about 6° along the fence, and Pluto takes years to cover 6°.
A body far from the hinge
One more run confirms the mechanism.
Run Pluto again at 270° instead of 180°, still carrying 6.28° of latitude:
| Body | Degree | Ecliptic latitude | Ecliptic crossing | Equatorial crossing | Difference |
|---|---|---|---|---|---|
| Pluto | 270° | +6.28° | 2008-01-25 21:38:04 | 2008-01-25 21:38:04 | none at all |
The two crossing times are identical to the second, although the body is well off the ecliptic.
The fence explains it in one line. 270° is as far from the hinge as it is possible to get. Out there the tilt lifts a hovering body straight up and down rather than swinging it sideways, so it stays over the same paling however high it is hovering. The same holds at 90°.
So the whole picture comes down to where the body is relative to the hinge:
| Where the body is | Effect of switching plane |
|---|---|
| On the ecliptic, at 0° or 180° | none — it is on the hinge |
| On the ecliptic, at 90° or 270° | none |
| On the ecliptic, in between | up to 2.5°, greatest near 45° |
| Off the ecliptic, at 0° or 180° | greatest — about 0.4 × latitude |
| Off the ecliptic, at 90° or 270° | none, at any latitude |
| Off the ecliptic, in between | both effects together |
A wrinkle worth knowing about the Moon
The Moon's row used 2030 rather than a nearer year, because of where the Moon's nodes are.
The Moon's latitude at any particular degree of longitude depends on where its nodes are, and the nodes travel right round the ecliptic every 18.6 years. In 2025 a node sat close to 0° Aries, so the Moon crossed that degree almost flat — and the two planes agreed to about ten minutes. By 2030 the node has moved, the Moon crosses 0° with better than 5° of latitude, and the gap has opened to four and a half hours.
For the same body at the same degree, the difference varies by a factor of twenty-five depending on the year, and neither figure is wrong.
What follows
Neither answer is wrong. They are answers to different questions. When did Pluto reach 180° of celestial longitude? and when did Pluto reach 180° of right ascension? are not the same question, and there is no reason they should have the same answer.
A result is incomplete without its plane. A transit date carries an implicit frame, and for a slow high-latitude body that frame is worth years. Two dates compared against one another must have been produced in the same reference plane or they are not comparable.
Ecliptic is the assumed frame. Tradition, every ephemeris and every other astrological program mean an ecliptic transit by an unqualified one, so a date produced without a deliberate change of frame agrees with all of them.
What comes next
The equatorial runs above reported plain degrees and no sign names: no
00°Lib00' anywhere in that output, where the ecliptic runs had it on every row.
That is deliberate, and it follows from everything on this page: the next page
explains why sign names are withheld under the Equatorial plane.